There are methods that can work as an insurance if you are worried about someone backing from a deal.

You and the other player can agree to have a Moderator act as a Middleman for the deal. This is a last resort if you have issues with trading directly to the player.

Notes:
1. Both players must agree to the middleman first. If one player has an issue with them, you have to use a different Moderator until both of you agree or decide not to trade with each other.

2. In this case, both you and the other player must send the same Moderator a PM on what they are supposed to send to the other player and what they are supposed to receive.

3. In this case, a Moderator will not buy the cards from their PS if BOTH players have not put the cards in.

4. A Moderator can ask queries if there is some issue between what is in their PS and what is mentioned in the PM as a consistency check.

5. Once a Moderator receives the cards, they will send it to the other player as instructed by the PM from both players.

Worst case scenario is that the Moderator is unable to help and it is as you said, you cannot get your card back. This can be managed by contacting Customer Support and identifying the player who scammed you. Do not identify it in a public forum as it will make things harder.

A player caught scamming another player for cards by not meeting their trading obligations is subject to blacklist and potentially having their market blocked.

There are cases of sales being reversed by Admins from what I heard if proven to be a scam or fraudulent. When a sale is reversed, you get the cards back.

Regarding the previous comment:
While a Moderator can be referred to, we do not exactly have the tools to get back cards or reverse sales immediately.
We can refer you to customer support, but the least we can do is take action against the player who scammed you by issuing a blacklist or talking about the issue with them.
What actually happens is Customer Support then reverses the sales to get back your cards or recover them.